1. Field of the Invention
The present invention relates in general to the use of satellite position signals to determine the position of a point on the surface of the earth and in particular to a system for accurately determining the position coordinates of a mobile GPS receiver utilizing a fixed reference station. Still more particularly, the present invention relates to a system for accurately determining the position of a mobile GPS receiver by resolving double difference GPS carrier phase integer ambiguity utilizing decentralized Kalman filters.
2. Description of the Prior Art
Since the early 1980's the Global Positioning System (GPS) satellite system has been utilized. This system will eventually comprise a large number of satellites in orbits approximately 11,000 miles above the earth's surface inclined about 55.degree. from the equatorial plane. The satellites are not at a constant position but have a twelve hour orbit. At any point on the earth a ground based receiver can normally receive signals from at least four GPS satellites. A basic explanation of GPS and its use in surveying is given in Hurn, "GPS, A Guide to the Next Utility," Trimble Navigation, 1989, incorporated herein by reference thereto.
Each GPS satellite transmits signals which contain information that enables distance measurement to be made by measuring the transit time of a pseudo-random number (PRN) code from a satellite to a GPS receiver. The PRN code is a very faint signal which hardly registers above the earth's natural background noise; however, this signal can be received by an antenna only inches in size. Decoding of these signals is accomplished in known fashion by sampling the PRN code and correlating the code with a replica code generated by a GPS receiver thus permitting the PRN code to be picked out of the earth's background noise.
The PRN code typically includes an implicit time signal, as measured by an atomic clock on board the satellite, at which the signal left the satellite. Over time, these signals also include information about the satellite's current orbit in space as well as corrections for known errors in the satellite's clock.
Two types of services produce signals from the GPS satellites. First, the Precise Position Service (PPS) produces for the military the most accurate dynamic positioning possible utilizing the GPS system, based upon the dual frequency Precise or Protected code know as the P-code. Users must have an encryption code in order to access the P-code which is not generally available to the public. Standard Positioning Service (SPS) produces the publicly accessible civilian positioning accuracy obtained by utilizing the single frequency "Clear Acquisition" (C/A) code. The Department of Defense has the ability to degrade the accuracy of the C/A code utilizing "Selective Availability" (S/A) or by artificially creating clock and other errors to prevent hostile military forces from navigating accurately utilizing the C/A code.
Computation of positional coordinates utilizing GPS signal data may be simply accomplished by receiving the PRN code and recording the received time as measured by the receiver's clock. Relative clock offsets may be taken into account and the difference between a signal's departure time and arrival time is the total travel time. The distance from a GPS satellite to the receiver's position may then be approximated by multiplying the speed of light times the total travel time. In this manner, if time is known, a position may be determined utilizing a minimum of three satellite signals. The calculated position can be at only one of two points at which three spheres around the three GPS satellites intersect. For a position known to be on earth, one of these points will generally be not possible (somewhere in space) so three satellites are generally enough to pinpoint a location. If precise time is not known then information from a fourth satellite will be necessary.
Thus, those skilled in the art will appreciate that GPS satellite broadcast systems permit PRN code tracking and analysis which permits positional fixes of high accuracy, but low precision due to refraction, clock errors, noise, time errors and ephemeris errors. Alternately, carrier phase-detection techniques may be utilized; however, such techniques produce high precision but low accuracy positional fixes due to uncertainties of carrier-wave identification and phase lock.
Solutions to enhance the carrier-phase estimation techniques generally involve measurement of carrier phase shift and an analysis of all likely numbers of integer waveforms which may have generated the resultant signal. This technique is rather time consuming in that all possible solutions for the number of integer waveforms must be analyzed and the most likely solution must then be determined.
Multiple techniques have been proposed for increasing the accuracy of the GPS position system and examples of these systems are set forth within U.S. Pat. Nos. 5,311,194, issued to Brown; 5,155,490, issued to Spradley, Jr., et al.; and 5,252,982, issued to Frei.
Upon reference to the foregoing those skilled in the art will appreciate that a method and system whereby the integer ambiguity of GPS carrier phase may be resolved in real time, such that a highly accurate position coordinates for a mobile GPS receiver may be determined, would be highly desirable.